Sessholvlaru got his Magnum to do over 195 with [IIRC] a used Nelson Q40 engine and whatever Q40 prop APC has to offer. I think he used a piece of plastic sheet to hide the lower half of the engine.
If he had "gone all the way" by trying some Eliminator props and spending more time at it, he might have come pretty close to 200.
My 25 year old Webra .50 is on it's 2nd set of parts. Those parts weren't cheap, but cheaper than buying another engine. I raised the exhaust to 180, lightened the piston, oval shaped the rod, swapped carb for a 11mm venturi and used a MAC's tuned pipe to make it run well with a 7.5 x 8 prop.
The results on a Screamin' Demon Delta were in the 170 mph range. I think the limiting factor for these small crankshaft "40 size" engines is that the cranks do not have a big enough hole going through them to process as much air as a Nelson type .40
I opened up the Webra crank as far as I dared with a "power hone" tool.
To take this any further, I'd have to do a side by side comparison by running this engine on a Magnum for a couple of weeks with a hand full of props and another exhaust system that Mac's might recommend to extract more rpm with FAI type props.